IIRC, I watched Alton Brown and he used a big cleaver, and then wacked it on top with a mallet. I can't recall if he used anything to keep the squash from moving around. Perhaps setting it on a folded towel would make it less apt to slip.

Yeh, those squash can be tough. I use my Wusthof 10 inch Chef knife which is pretty heavy. Work it in first in the middle of the knife then push down hard on both ends while keeping my fingers out off the way!